1) Field of the Invention
The present invention relates to an optical module that has an optical fiber and a laser diode optically coupled together, and includes a monitoring photodiode that receives a beam emitted from the back side of the laser diode.
2) Description of the Related Art
In the optical communications, an optical fiber amplifier, which employs an EDF (Er-doped optical fiber), is used for amplifying the optical signal transmitted through the optical fiber. This optical fiber amplifier has an optical module as a light source and, amplifies the light generated by the optical module. FIG. 7 shows a structure of a conventional optical module 100. A reference numeral 1 denotes a laser diode (LD) that emits a laser beam from the front side (front laser beam) and the back side (back laser beam) thereof, and 2 denotes a lens that focuses the front laser beam. A reference numeral 3 denotes an optical fiber that transmits the laser beam focused by the lens 2, and 4 denotes a monitoring photodiode (PD) that receives the back laser beam and outputs a current proportional to the received laser beam. A reference numeral 50 denotes an output terminal-from where the current (monitoring current) of the photodiode 4 is output to the outside. A reference numeral 5 denotes an APC (Automatic Power Control) circuit that controls the output of the laser diode 1 according to the monitoring current and, it is a part of a not shown optical transmitter.
The conventional optical module 100 operates as follows. The laser diode 1 emits the front laser beam and the back laser beam. The lens 2 focuses the front laser beam. The optical fiber 3 transmits the laser beam focused by the lens 2 to the outside. The back laser beam falls on the photodiode 4. The photodiode 4 outputs the monitoring current. The APC circuit 5 receives the monitoring current via the output terminal 50. The APC circuit 5 adjusts or controls the power provided to the laser diode 1 based on the monitoring current.
One of the causes of the tracking error is the deviation of the optical axis (optical axis deviation). To transmit the laser beam effectively, it is necessary that the laser beam from the lens 2 falls at the center of the core of the optical fiber 3. The optical axis deviation is a positional difference between the focal point of the lens 2 and the point (beam input point) on the end surface of the optical fiber 3 where the laser beam focused by the lens 2 falls. The optical axis deviation may occur due to, for example, a change in the temperature inside the optical module 100. The optical axis deviation lowers the quantity of the beam incident to the optical fiber, which results in a reduction in the beam output to the end surface of the optical fiber. This reduced quantity of the output beam becomes a factor of the tracking error. The tracking error causes a variation in the gain of the optical fiber amplifier, and this becomes a serious problem in the optical communications.
FIG. 8 shows a relationship between the monitoring current and the output of the optical fiber 3 (fiber-end beam output). When the monitoring current is 0.155 mA, the output at 5° C. is 109.5 mW, and the output at 45° C. is 96.5 mW. Therefore, the tracking error of −0.55 dB (=10×LOG (96.5/109.5)) occurs.
FIG. 9 shows relationships between the output and the optical axis deviation (dotted line) and, between the monitoring current and the optical axis deviation (continuous line). The output is considerably lower when there is the optical axis deviation, however, the monitoring current is not much affected by the optical axis deviation. This is because the back laser beam becomes a scattering beam because there is no lens between the laser diode 1 and the photodiode 4. On the other hand, the front laser beam is focused by the lens 2. Even if the optical axis deviation is small, of the order of micrometers, the laser beam is not focused at the center of the optical fiber 3 and therefore, the output is lowered.
Thus, in the conventional optical module, the monitoring current is output from a scattered laser beam that is not much affected by the temperature. Since the APC circuit performs the power control based on such monitoring current, there is a problem that the tracking error becomes large.